Access networks are presently experiencing double-digit growth in the United States, Europe, and a number of Asian countries. Residential and business customers are demanding higher bandwidth from their Internet service providers for multimedia services IPTV, telephony, and high-speed Internet. As a result, many service providers are planning to implement networks capable of delivering 100 Mb/s and higher bandwidths per customer. A number of access network architectures have been developed to address this growth. The cost of implementing any network technology plays a critical role in the decisions related to its adoption and deployment. Passive-optical-networks (PON) feature lowest capital-equipment expenditures relative to point-to-point and active optical networks. In a PON, the remote nodes (RN) between the feeder fibers (FF) connecting to the central office (CO) and the distribution fibers (DF) connecting to different optical network units (ONU) are passive. Examples of such networks are shown in, for example, the book by C. F. Lam, Passive Optical Networks Principles and Practice, Academic Press, 2007, and publication by C-H. Lee, W. V. Sorin, and B. Y. Kim, “Fiber to the Home Using a PON Infrastructure”, IEEE J. Lightw. Technol., vol. 24, no. 12, pp. 4568-4583, 2006. The use of wavelength division multiplexing in passive optical networks (WDM-PON) is actively investigated as a next-generation optical network architecture meeting the future cost and performance needs due to (a) its point-to-point capability, where it has an advantage over time-division multiplexing passive optical networks (TDM-PON), (b) high-degree of privacy (each user receiving own wavelength), and (c) data rate upgrades available on each channel independently. Most importantly, WDM-PON provides higher bandwidth per user than any other network architecture and hence potentially offers the lowest cost per unit of bandwidth to the user. The key difficulty in such a system has been the cost of the components, particularly arising from the need to transmit light at one wavelength for a specific channel and also receive information at any one of several other wavelengths in the ONU. WDM optical and optoelectronic components traditionally exhibit high cost, among other issues, due to precise wavelength definitions in such systems. One attempt to eliminate the need for wavelength-specific transceivers at the ONU is the introduction of colorless WDM-PON systems.
In a colorless optical network, the wavelengths emitted and received by the transceiver in the ONU are defined in the RN or the CO rather than in the transceiver at the ONU. Such a system can use identical wavelength-nonspecific ONU transmitters, which are significantly less expensive to produce than ONU transmitters in which the wavelengths have to coincide with the DWDM grid specified by the International Telecommunications Union (ITU) organization. A wavelength-nonspecific transmitter can be realized as an injection-locked Fabry-Perot laser, in which the injected wavelength is defined by a laser located at the CO, or by seeding a reflective semiconductor optical amplifier (RSOA) at the ONU using a spectrally sliced broadband light source, such as an LED or Erbium-Doped-Fiber-Amplifier (EDFA) based amplified spontaneous emission (ASE) source and an athermal array-waveguide grating (AWG) [4],[6]-[9]. When the same wavelength is used for downstream and upstream communications, the signal is re-modulated in the RSOA. These approaches still suffer from high cost components: the ASE sources and wavelength specific sources for injection-locking. Further reduction in complexity and cost can be realized by removing the need for centralized injection locking or a seeding source, by using a reflector at the remote node to allow the ASE signal emitted from the RSOA to seed itself. The first experimental demonstration of an RSOA self-seeding architecture was shown in the publication by E. Wong, K. L. Lee, T. B. Anderson, “Directly Modulated Self-Seeding Reflective Semiconductor Optical Amplifiers as Colorless Transmitters in Wavelength Division Multiplexed Passive Optical Networks”, IEEE J. Lightw. Technol., vol. 25, no. 1, pp. 67-74 (2007). However, in that work, insufficient seeding light power was observed to lead to an undesirably high bit-error floor, while the modulation present in the seeding light reduced the eye opening in the upstream beam.
Therefore, an unmet need for a low-cost high-performance WDM-PON solution exists in the industry. This application discloses a network system architecture and innovative components of that network system that enable cost reduction and performance improvement relative to the existing technology offerings.